The landgraves of Hesse-Kassel lost their principality in 1866 to Prussia. The line of Hesse-Darmstadt however died out with prince Louis V of Hesse and of the Rhine on 30th May 1968. As he had adopted Moritz of Hesse-Kassel on 24th December 1960, the line of Hesse-Kassel is now also the grand ducal line in Hesse(-Darmstadt). Similarly to the [federal] state of Hesse, they use the lion without the sword [or the crown]. Source: Ströhl 1897.

The landgraves of the Hesse-Kassel branch flew the same flag [as the Hesse-Darmstadt royal family] with just the shield, but with a golden border and the lion doing without the sword. Source: Ströhl 1897.

Theo van der Zalm, 24 August 2000

Presumably this standard was used before 1866 by the Elector of Hesse. The (black and white, hatched) image in Ströhl 1897 shows the golden bordure reaching out of the white stripe on the top and bottom well into the red ones. Ströhl 1897 shows this flag with a ratio of approximately 5:8.